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Signatures of the fast dynamics in glassy polystyrene: First evidence by high-field Electron Paramagnetic Resonance of molecular guests
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10.1063/1.2085027
/content/aip/journal/jcp/123/17/10.1063/1.2085027
http://aip.metastore.ingenta.com/content/aip/journal/jcp/123/17/10.1063/1.2085027
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Schematic view of the bimodal distribution of correlation times [Eq. (8) with ] for different values of the trapped fraction . , , denotes the shortest correlation time. The delta function is replaced by a narrow Gaussian with a width of 0.01.

Image of FIG. 2.
FIG. 2.

Calculated EPR line shapes at 190 GHz of a nitroxide spin probe for different jump angles . SCT model [Eq. (5)]. Top: from the top to the bottom the correlation times are , , , , , , and . Bottom: from the top to the bottom the correlation times are , , , , , , and . The magnetic parameters are , , , , , and . The axis is parallel to the N–O bond, the axis is parallel to the nitrogen and oxygen orbitals containing the unpaired electron, and the axis is perpendicular to the other ones (see Fig. 4 for details). Each curve is convoluted with a Gaussian with a width of 0.15 mT. The vertical lines on the top panel mark the positions of the maxima of the outermost peaks at the slowest relaxation rate. They help the reader to appreciate the shifts of the maxima when the reorientation rate increases.

Image of FIG. 3.
FIG. 3.

Dependence of the distance between the outermost extrema of the HF-EPR line shape at 190 GHz (see Fig. 2) on the rotational correlation time for small and large jump angles .

Image of FIG. 4.
FIG. 4.

Chemical structures of PS and the spin probe TEMPO.

Image of FIG. 5.
FIG. 5.

Temperature dependence of the quantity at 190 and 285 GHz of TEMPO in PS (see Fig. 2 for the definition). Continuous line: linear fit with , , . Dashed line: guide for the eye. Inset: average linewidth of the three outermost lines on the right side of the line shape (see Fig. 2).

Image of FIG. 6.
FIG. 6.

The line shape at 190 GHz (a) and 285 GHz (b) of TEMPO in PS at 50 K. The superimposed dashed lines are best fits according to the SCT model, Eq. (5), with (190 GHz) and (285 GHz). Jump angle . Nearly identical agreement is obtained by decreasing the jump angle down to with (190 GHz) and (285 GHz). Magnetic parameters of TEMPO as in Fig. 2. The theoretical line shape is convoluted by a Gaussian with a width of 0.15 mT to account for the inhomogeneous broadening.

Image of FIG. 7.
FIG. 7.

The HF-EPR line shape of TEMPO in PS at 180 K and frequencies 190 GHz (a) and 285 GHz (b). The dotted superimposed lines are simulations by using the SCT model with jump angle and (a); (b). The dashed superimposed line in panel (a) is a simulation using the SCT model with jump angle and . Magnetic parameters of TEMPO as in Fig. 2. The theoretical line shape is convoluted by a Gaussian with a width of 0.15 mT to account for the inhomogeneous broadening.

Image of FIG. 8.
FIG. 8.

The EPR line shape at and frequencies 190 GHz (a) and 285 GHz (b). The dotted lines are numerical simulations by using the TPD model [Eq. (8) with , Eq. (4)] with , (a); , (b). For each frequency at 180 K and , see Fig. 7. From Eq. (9) the fraction of TEMPO molecules undergoing not activated motion was and . The dashed lines are numerical simulations by using the SCT model with [panel (a)] and [panel (b)]. In both cases the best-fit value of the jump angle is . Notice that the TPD model has only one more adjustable parameter with respet to SCT one. Magnetic parameters of TEMPO as in Fig. 2. The theoretical line shape is convoluted by a Gaussian with a width of 0.15 mT to account for the inhomogeneous broadening.

Image of FIG. 9.
FIG. 9.

Temperature dependence of the fraction of trapped TEMPO molecules, Eq. (9).

Image of FIG. 10.
FIG. 10.

Best fit of the EPR line shape using SCT, LGD, and PD from 190 GHz, 270 K, and a jump angle . For SCT: ; for LGD: , ; for PD: , . Magnetic parameters of TEMPO as in Fig. 2. The theoretical line shape is convoluted by a Gaussian with a width of 0.15 mT to account for the inhomogeneous broadening.

Image of FIG. 11.
FIG. 11.

Temperature dependence of the width of the exponential energy-barrier distribution, Eq. (3), as detected by the EPR at 190 GHz (squares) and 285 GHz (triangles). Previous measurements by internal friction (Ref. 31), Raman (Ref. 18), and light scattering (Ref. 28) yield , and , respectively.

Image of FIG. 12.
FIG. 12.

Temperature dependence of the characteristic times of the SCT, PD, and TPD distributions. The error bars at 50 and 180 K account for the uncertainty on the best-fit value of the jump angle which is in the range of and , respectively. The dotted lines are guides for the eye.

Image of FIG. 13.
FIG. 13.

The exploration of the orientational energy landscape by TEMPO. : all molecules are trapped (). Orientation correlations are lost via nonactivated entropiclike pathways. : a fraction of the molecules equal to rotate by activated jumps over the exponentially distributed energy barriers.

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/content/aip/journal/jcp/123/17/10.1063/1.2085027
2005-10-31
2014-04-16
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Signatures of the fast dynamics in glassy polystyrene: First evidence by high-field Electron Paramagnetic Resonance of molecular guests
http://aip.metastore.ingenta.com/content/aip/journal/jcp/123/17/10.1063/1.2085027
10.1063/1.2085027
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